(A lighthearted, short adventure for a small party of low-level characters with dark overtones: Dustmen, undead, lawful evil or neutral evil characters who might be found near Hopeless, servants of deities with death in the portfolio, or the like.)
Hades, dark god of the Greek underworld, has at least one soft spot in his heart, for his wife Persephone. The daughter of the goddess of agriculture spends half the year pining away in his halls, and outside the orchards sag listlessly in the land of the dead. News has come to Hades' ears of a wondrous fruit that banishes sadness when eaten and even lifts spirits by the odor of its vines: it has been bred in the Outlands garden of Sheela Peryroyl, halfling goddess of agriculture. His priests have been ordered to bring several living cuttings to Hades' realm for planting, to please Persephone.
Personally, they wouldn't be caught dead in a realm called Flowering Hill, and can easily foresee trouble if the representatives of a power with such a diametrically opposed portfolio waltzed in casually, so they've stopped into Hopeless, or Sigil (wherever the characters are) to hire some living berks to do it. It should be an easy job, and they don't need any really experienced bloods: all you have to be is willing to make a journey first to Sheela's realm with enough coin to buy a few of the things and then to some representatives in Hopeless to pass the goods on (or to Hades itself, if you like -- money's no object for the god of wealth). The priests will naturally prefer to hire like-minded folk: adventuring Dustmen or undead will do nicely, though clerics of any deity with death in the portfolio will also work. In a pinch, they'll take evil bashers sufficiently lawful to stay bought and offer them the chance to keep any of the price they don't spend, over the basic commission. (Set a price, based on the rumored potency and rarity of the fruits, to make this a reasonably tempting offer for the characters.) Not being very trusting bashers, they'll use Zone of Truth and ask the party to "fully describe your intent in this agreement" to seal the deal before they hand over the money.
Since no gate goes directly from Sigil (or Hopeless, of course) to Sheela's realm, the party will need to cross the Outlands to Flowering Hill, which is just Spireward of Elysium. There's plenty of room for random Outlands encounters, as well as more targeted encounters with the righteous sorts of folk that wander around the upper half of the Great Ring. If in their preparations or on their travels they're loose-tongued about the price they're carrying, banditry is an excellent option.
When they arrive at Flowering Hill, they'll shortly be greeted by bounders (halfling border patrol) and be asked their business, especially if they look like trouble (hint: most traveling adventuring parties do). The opinionated locals will also put their two cents in; the interplay between the feisty, nature-loving halflings and the death-embracing types ought to be fun.
The bounders won't have heard of the plant and will direct the party to the organized clergy in the temple at a nearby halfling village. (This is assuming they haven't initiated any violence, which will get them kicked out of the realm -- whereupon they'll have to employ sneak-thieving to get the plant or the necessary information. Of course, some parties may have already had this idea, and/or see this as entitling them to the entire purchase price as "commission"...)
If the plant really exists, they'll need to convince the clergy to sell them a few cuttings. Money isn't as attractive an explanation as the opportunity to do something nice for Persephone -- they've certainly no love for Hades or his dark realm. Then, the party can deal with the annoyingly uplifting effects of the plant and theft attempts as they bring it back to Hopeless or the Waste. Again, theft attempts for someone obviously bearing a live, magical plant are possible. Furthermore, the plant is a rich green and always has brightly colored flowers -- the natives of Hopeless react badly to such things, and will crazedly attempt to destroy it. The party will need to hold off a number of unskilled but determined nutcases long enough to find their contacts in Hopeless or make it through the gate to Hades. If they're taking the plant that far themselves, it will also need to be kept alive through the trip across the Waste, which poses the usual panoply of dangers. (The party may need to be rather higher level to attempt this.)
On the other hand, at the temple the couriers may learn that the rumors are screed... the priests have heard the stories, but no such plant exists. They certainly seem like they're not lying; looks like the group will have to return empty handed and explain the affair to their employers. As they head back, though, they are stopped on the road by one of Sheela's higher-ranking clerics. He has received a brief message via spell that Sheela pities Persephone and could make such a plant, if the PCs would be willing to undertake a certain mission they're a tad more suited for than her usual followers...